NRF52840 USB VCC requirements

RE: NRF52840 USB VCC requirements

lo — Wed, 13 Nov 2019 17:22:14 GMT

ok, thank you for the confirmation.

RE: NRF52840 USB VCC requirements

Sigurd — Wed, 13 Nov 2019 17:01:44 GMT

No, this is out-of-spec as well, and not how it's designed to be used. Worst-case you risk damaging the chip.

RE: NRF52840 USB VCC requirements

lo — Wed, 13 Nov 2019 16:00:40 GMT

OK, thank you for the confirmation, however what i wonder is wether we can power the USB core directly via DECUSB and what will append with the USBDETECT if we do so.

RE: NRF52840 USB VCC requirements

Sigurd — Wed, 13 Nov 2019 15:38:38 GMT

Hi,

As mentioned by TurboJ, the VBUS USB supply voltage must be within 4.35V and 5.5V. Voltages outside this range is then out of spec, this is not something we can recommend and correct behavior will not be guaranteed.

RE: NRF52840 USB VCC requirements

lo — Wed, 13 Nov 2019 14:53:46 GMT

an official statement from Nordic would be welcome on this issue, since the design of the M.2 breakout has to be finished today and the possibility of voiding a DC boost on the board is a big deal for us.

RE: NRF52840 USB VCC requirements

lo — Wed, 13 Nov 2019 08:42:39 GMT

Yes i totally agree that, even if 3.4V was enought it would likely never be reached anyway. So i think our only option is to add a DC boost on the M.2 breakout.

RE: NRF52840 USB VCC requirements

Turbo J — Tue, 12 Nov 2019 22:54:14 GMT

An USBDETECTED voltage of about 4V won't help much you if there is only 3.3V available.

Remember statistics? I would expect a gaussian distribution - many chips with around 4V USB detect threshold, and very very few at the extremes (3.4 and 4.3V).

RE: NRF52840 USB VCC requirements

lo — Tue, 12 Nov 2019 22:13:10 GMT

Thanks, i have seen that requirement, but i have also seen that VBUS directly goes to an LDO and the PHY itself is actually powered by 3.3V:

The USB peripheral has a dedicated internal voltage regulator for converting the VBUS supply to 3.3 V used by the USB signalling interface (D+ and D- lines, and pull-up on D+). The rest of the USB peripheral (USBD) is supplied through the main supply like any other on-chip feature.

That is what lead me to think it may be possible to power the USB PHY directly with 3.3V from DECUSB which is just after this LDO, the only problem would be the VBUS detection, which according to datasheet seems to be done before the LDO with a minimum of 3.4V.

VBUS,DETECT Voltage at which rising VBUS gets reported by USBDETECTED 3.4 - 4.0 - 4.3 V

RE: NRF52840 USB VCC requirements

Turbo J — Tue, 12 Nov 2019 21:58:26 GMT

Nope. V_Bus minimum is 4.35V, see Power chapter in the PS. Lower voltage will most likely not power up the PHY properly (USBD power up sequence in the PS requires valid V_Bus).